1. Field of the Invention
The present invention relates to a semiconductor device, which uses a semiconductor film having a crystal structure, and a manufacturing method of the semiconductor device. More particularly, the present invention relates to a semiconductor device typified by a thin-film transistor (hereinafter referred to as the “TFT”) and a manufacturing method of the semiconductor device. It should be noted here that in this specification, the term “semiconductor device” refers to the whole of devices that operate by utilizing semiconductor characteristics.
2. Description of the Related Art
In order to form an integrated circuit using TFTs, there has been attached importance to a technique of forming semiconductor films having crystal structures on an insulating surface. This is because the semiconductor films are used to form active layers of TFTs (in this specification, the active layers include channel forming regions, source regions, and drain regions) and therefore, the quality of the semiconductor films is a factor that directly determines the electric characteristics of the TFTs.
As a method of forming a semiconductor film having a crystal structure, there is used a method with which crystallization is performed through the irradiation with laser light after an amorphous semiconductor film is formed. Alternatively, there is used a method with which crystallization is performed through a heating process using an electric heating furnace. However, a semiconductor film produced using these methods is composed of a plurality of crystal grains and the crystal orientation thereof is oriented in an arbitrary direction. Therefore, it is impossible to control the crystal orientation. This becomes a factor that hinders the smooth movement of carriers and limits the electric characteristics of a TFT, in comparison with the case of a single crystal semiconductor.
In view of this problem, Japanese Patent Application Laid-open No. Hei 7-183540 discloses a technique with which a silicon semiconductor film is crystallized through the addition of a metallic element such as nickel. It is publicly known that this technique achieves an effect that crystallization is promoted by the metallic element functioning as a catalyst and the temperature required for the crystallization is decreased. In addition to this effect, it also becomes possible to enhance the orientation property of crystal orientation. It is publicly known that an element having the catalytic action is one type or a plurality of types selected from the group consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.
However, the addition of such a metallic element having the catalytic action (in this specification, every metallic element having the catalytic action is referred to as the “catalytic element”) causes various problems, such as a problem that the metallic element remains within a semiconductor film or on the surface of the semiconductor film and causes variations of electric characteristics of a TFT. For instance, there occurs a problem that the off-current of a TFT increases and there occurs differences in off-current between respective devices. That is, once a crystalline semiconductor film is formed, the metallic element having the catalytic action in terms of crystallization becomes a substance that is contrarily unnecessary.
With a gettering technique using phosphorus, it is possible to remove the metallic element added to perform crystallization from a specific region of a semiconductor film even at a heating temperature of around 500° C. For instance, by performing a heating process at 450 to 700° C. after the addition of phosphorous to the source region/drain region of a TFT, it is possible to remove the metallic element added to perform crystallization from a device forming region without difficulty. An example of this technique is disclosed in Japanese Patent No. 3032801.
By the way, as the gettering technique, there have been known extrinsic gettering and intrinsic gettering. In the extrinsic gettering, a gettering effect is achieved by giving a strain field or chemical action to a silicon wafer from the outside. On the other hand, in the intrinsic gettering, the gettering effect is achieved by utilizing a strain field caused by lattice defects due to oxygen generated within a wafer. As the extrinsic gettering, there have been known a method with which mechanical damage is given to the back surface (a surface on the side opposite to a surface on which a device is to be formed) of a silicon wafer, a method with which a polycrystalline silicon film is formed, a method with which phosphorous is diffused, and the like. There is also known a gettering technique with which a strain field is formed using secondary lattice defects resulting from ion implantation. These techniques have evolved until today as a technique of manufacturing a large scale integrated circuit using a single crystal silicon substrate and have been developed based on the premise that a silicon wafer is used. There are included experimental elements in the developed techniques. At all events, the gettering is a technique with which metallic impurities and the like contained in a semiconductor are accumulated in a predetermined region (a gettering site) by moving the metallic impurities and the like with any energy, thereby reducing the concentration of the metallic impurities in a device forming region (a gettering target region).
Phosphorous is used as a donor in a wide variety of semiconductor devices to form n-type semiconductor regions and is an element that is known as a dopant. Accordingly, it is possible to incorporate gettering using phosphorous into a manufacturing process of TFTs with relative ease. The gettering using phosphorous makes it possible to remove a metallic element introduced into a semiconductor film for the crystallization of silicon by performing a heating process at 550° C. for around four hours. However, this in turn causes a problem that the concentration of phosphorous that needs to be added to a semiconductor film is 1×1020/cm3 or higher, preferably 1×1021/cm3, and therefore the processing time required to perform doping is elongated. Further, the addition of phosphorous with an ion implantation method or an ion doping method (referring to a method with which there is not performed mass separation of ions to be implanted, in this specification) brings about an amorphous state of a semiconductor film and the addition of dense phosphorous causes the difficulty of recrystallization afterward.